Commercially available micromechanical membrane sensors based on silicon technology are usually implemented in the form of thin-film membranes, produced by first depositing layer systems in thicknesses between a few tens of nanometers and a few micrometers as the membrane layer and then removing the supporting substrate beneath it in the areas where the membrane is to be unsupported. Such membranes are often used for thermal separation of sensor elements arranged on the surface of the membrane from surrounding components arranged on the supporting substrate made of silicon.
Such a thermal membrane sensor and method of producing same is proposed in German Published Patent Application No. 197 52 208, for example, on which the present invention is based. In particular, it describes first depositing a thin layer of silicon carbide, silicon oxide or silicon nitride over an area of porous silicon formed in the surface of a silicon substrate. Then by a dry etching method, openings which extend down to the porous silicon layer are formed in this silicon carbide or silicon nitride layer. After arranging printed conductor structures, semiconductor components or thermocouples on the surface of the unsupported membrane to be formed, finally the sacrificial layer of porous silicon beneath the unsupported membrane to be produced is removed with a suitable solvent such as ammonia, KOH or TMAH. This forms a hollow space beneath the membrane layer, thermally isolating the sensor membrane from the surrounding substrate.
In addition to the method according to German Published Patent Application No. 197 52 208 for exposing the membrane, i.e., using a sacrificial layer which is produced before depositing the membrane on the front side of the substrate and then is removed later from the front side of the substrate through producing openings in the membrane by dissolution, it is also known that the membrane to be exposed can be exposed from the backside of the substrate by etching the entire substrate from the backside until forming the desired unsupported membrane on the front side. Both known methods, which differ fundamentally, are assigned to the categories of surface micromechanics (SMM) and bulk micromechanics, respectively.
Porous silicon is a possible sacrificial layer for a surface micromechanical etching process to produce a largely unsupported membrane such as that known from German Published Patent Application No. 197 52 208. Production of porous silicon is based on an electrochemical reaction between hydrofluoric acid and silicon in which the silicon is connected as the anode with respect to the hydrofluoric acid electrolyte, producing a spongy structure in the silicon. Due to this porous structure, the silicon then has a large internal surface area and thus has different chemical properties from the surrounding silicon. For example, the reactivity of porous silicon is greatly increased in comparison with that of bulk silicon, thus permitting a selective dissolution process.
It is known that doped silicon substrates are suitable for the production of porous silicon, generally using p-doped silicon substrates with different degrees of doping for this purpose. The structure size inside the resulting porous silicon can be defined by the degree of doping, i.e., it is possible to form both nanoporous silicon having a pore size between 1 nm and 5 nm and mesoporous silicon having a pore size is between 5 nm and 50 nm due to a high p-doping.